Switching power converters offer higher efficiency as compared to linear regulators. Although linear regulators are relatively inexpensive, they regulate a lower output voltage from a higher input voltage by simply burning the difference as heat. As a result, a linear regulator typically burns more power than is actually supplied to the load. In contrast, a switching power converter regulates its output voltage by delivering relatively small increments of energy through the cycling of a power switch. The power switch in a switch-mode device is either off or on such that efficiency is markedly improved as compared to linear regulators. Switching power converters are thus typically used to regulate the power supply voltage for a mobile device as the resulting increased efficiency extends battery life.
A common type of switching power converter that may be used to step down an input DC voltage into an output DC voltage is a buck converter. In a buck converter, a power switch coupled to an inductor is cycled on so that an input current is driven into an inductor, which thereby stores energy. When the power switch is cycled off, the stored energy is delivered to a load as an output current from the inductor. Inductors cannot tolerate an abrupt ceasing of their current such that a buck converter requires some means of allowing the inductor to deliver its output current after the power switch is cycled off. In an asynchronous buck converter, a diode becomes forward biased in response to the cycling off of the power switch to allow the inductor to deliver its output current to the load. In contrast, a synchronous buck converter replaces the diode with a low side switch transistor. The power switch is denoted as the high side switch in a synchronous architecture. The lower resistance of the low side switch as compared to a diode provides synchronous buck converters with greater efficiency.
A controller in a synchronous buck converter controls the high side and low side switch cycling to regulate the power delivery to the load. The controller requires some sort of feedback from the load to maintain the desired regulation. It is thus conventional to use a sense resistor in series with the low-side switch to measure the output current and regulate accordingly. But such direct sensing increases power dissipation, size, complexity, and cost. Thus, indirect sensing architectures have been developed in which the drain-to-source voltage of the low-side switch is measured during an on cycle. The controller may then multiply the sensed voltage with a presumed value of the drain-to-source on resistance of the low-switch in that is denoted as an indirect sensing regulation scheme. But there is considerable variation from device to device in the drain-to-source on resistance. Moreover, this resistance also varies with temperature such that indirect sensing switching power converters suffer from inaccurate regulation.
Accordingly, there is a need in the art for improved ways of sensing the load current in a switching power converter.